The present invention relates to wastewater treatment systems and, more particularly to a process that includes pretreating digested sludge so as to increase the efficiency of sludge dewatering and to produce a relatively clean dewatered sludge that can be economically transported.
Activated sludge systems are widely used throughout the world to treat wastewater, with the intent of significantly reducing the concentrations of certain nitrogen and phosphorous containing compounds as well as generally reducing BOD levels within the wastewater. See, for example, U.S. Re. Pat. Nos. 32,429 and 4,874,519, the disclosures of which are expressly incorporated herein by reference.
Activated sludge systems of the type contemplated herein typically receive a stream of raw wastewater influent which is directed through a sequence or series of treatment stages. These treatment stages typically include aerobic, anaerobic, and/or anoxic processes. Upon completion of these treatment stages, the processed wastewater stream is passed to a final clarifier stage where the residual solid sludge particulate matter is physically separated and removed from the wastewater. The resulting purified effluent may then be discharged into a lake or stream, while a portion of the sludge that has been isolated by the final clarifier is returned or recycled to the head of the activated sludge system. The residual or non-recycled portion of sludge, which is often referred to as waste activated sludge (WAS), is typically directed from the final clarifier for disposal as an agricultural resource, such as a fertilizer or soil additive.
However, prior to release or distribution as an agricultural fertilizer, all contaminants must first be removed or separated from the WAS as well as other sludge produced during the waste water treatment process. Therefore, in a typical activated sludge system or process, the WAS component of the waste material stream as well as other sludge, such as primary, is directed to a digester where the sludge is further treated and cleaned by removing various pathogens and organic matter or volatile elements.
With particular regard to the digestion process cited above, there are a variety of sanitary levels or degrees that can be achieved through the use of such treatment processes. For example, some wastewater treatment facilities require a very clean grade of sludge product, so that the material may be disposed on agricultural fields or in sanitary landfills. Such digested sludge material is often referred to as Class A sludge. It should be noted, however, that there are other lower grades or classes of sludge which are not as clean or pure as a Class A sludge product. One type of digester, currently in wide commercial use, is an auto thermal thermophilic aerobic digestion system (ATAD). In this process, the incoming waste activated sludge is subjected to a mechanical thickening process and then stored in a holding tank for eventual metering to and processing by the ATAD system. The ATAD system typically consists of one to three serially-connected reactors in which the sludge undergoes an aerobic digestion process.
In general, the relative water content of the final sludge product varies depending upon the desired end use or disposal method. The sludge material stored in a post-ATAD holding tank typically contains a significant amount of water and is often used for such applications as roadside watering. However, if the sludge product is to be used as a fertilizer or soil additive, a reduction in water content is generally desirable so as to reduce the costs associated with transporting the waste material from the treatment facility to the agricultural site. Such a water content reduction is typically achieved via a post ATAD dewatering process, so as to produce a more concentrated final sludge product. Following dewatering processing, the concentrated sludge product assumes the consistency of a damp soil which may be efficiently transported and effectively used as a fertilizer on agricultural fields or safely disposed of in a solid waste landfill.
Dewatering of the digested sludge or treated biosolids material is currently accomplished through the use of mechanical means, such as a centrifuge for example. Typically, polymer additives are used in conjunction with such centrifugal dewatering devices to facilitate and expedite flocculation of the suspended biosolids. Such polymer additives are able to produce the desired flocculating effect as a consequence of the fact that the solid or particulate component of the treated wastewater solution is generally of an anionic nature, while the polymer additive exhibits a multivalent, cationic character. Thus, once introduced into the treated wastewater solution, each multivalent cationic polymer molecule will typically bond to several neighboring anionic biosolid particles, and as a consequence will effectively cause the biosolids component of the sludge to floc or aggregate together into physically larger particles. In general, such enlarged particle aggregations of flocs are easier to isolate and separate from the wastewater solution when using centrifugal dewatering techniques. It should also be noted that bonding of the sludge with the polymer agent, has the added benefit of generally driving water out of the resulting flocs thereby promoting the dewatering process.
It will be appreciated that a typical volume of wastewater will naturally contain small quantities of a number of multivalent cations, such as Ca+2 and Mg+2, which will tend to induce the same flocculating effects as the synthetic polymer additive. However, there are also typically small quantities of monovalent cations, such as NH4+1 (ammonia nitrogen), present in the wastewater which tend to compete with the floc inducing multivalent cations for binding sites present on the anionic biosolid particulates. Being monovalent, these cations are not able to effectively bind to multiple biosolid particles simultaneously, and hence do not facilitate flocculation of the biosolids material. Therefore, it will be appreciated that the presence of monovalent cations in the wastewater solution tends to reduce the overall efficiency and effectiveness of the flocculating polymer additives.
An additional inefficiency, with regard to the use of flocculating polymer additives, involves the inherent presence of protein and polysaccharides compounds as well as COD in the wastewater solution. These naturally occurring, organic compounds tend to interact or react with the flocculating polymers in such a manner so as to effectively neutralize or significantly reduce the polymer""s floc inducing qualities.
Obviously, reducing or minimizing the impact of these factors which compete with or diminish the effectiveness of flocculating polymers with tend to increase the overall cost effectiveness of the dewatering process. Therefor, with particular regard to dewatering processes which utilize flocculating polymer additives, there is and continues to be a need for a method of reducing the competing and generally disruptive effects of monovalent cations, proteins, and polysaccharides compounds typically found in the waste water solution.
The present invention entails a method of conditioning and dewatering thermophilic aerobically digested sludge. Prior to dewatering, iron or aluminum along with a cationic polymer is mixed with the digested sludge. The addition of the iron or aluminum significantly reduces the polymer demand for acceptable dewatering and consequently reduces the overall cost of the dewatering process. In one embodiment, the iron or aluminum is added in the form of a salt such as ferric chloride or aluminum. In one embodiment of the present invention, the digested sludge is subjected to mesophilic aeration prior to being treated with iron or aluminum. The aeration of the sludge acts to nitrify the ammonia nitrogen found within the sludge and evidence suggests that the aeration decrease the protein concentration in the sludge. As will be appreciated from subsequent portions of this disclosure, ammonia nitrogen and protein tend to increase the polymer demand for acceptable dewatering. Thus by nitrifying the ammonia nitrogen and reducing the protein concentration, the dewatering characteristics of the sludge is enhanced.
It is therefore an object of the present invention to provide a sludge conditioning process that will reduce cationic polymer demand for dewatering autothermal thermophilic aerobic digested sludge